Herpetology Notes, volume 10: 443-448 (2017) (published online on 05 September 2017)

Diet of post-metamorphic Rhinella icterica (Spix, 1824) from the Araucaria Plateau of Rio Grande do Sul, Brazil (Anura: Bufonidae)

Mirco Solé1,*, Matheus S. Rocha2, Cecilia Decarli2, César R. Santos2 and Clarissa K. Pereira2

Abstract. The toad Rhinella icterica (Spix, 1824) is distributed in the Atlantic Rainforest of Brazil but can also be found in Paraguay and Argentina. Few studies have reported the diet of adults of this species and none the diet of post-metamorphs. In this study we analyzed stomach contents of post-metamorphic R. icterica in order to assess the composition of ingested prey and the relation between toad body size and the volume retrieved from the stomachs. We collected 45 post-metamorphs at the Centro de Pesquisa e Conservação da Natureza – Pró-Mata, Rio Grande do Sul, Brazil. Stomach contents were recovered from toads via stomach flushing after measuring and weighing. The retrieved prey clump was also weighed and the 1293 prey items were classified into 13 categories at the lowest possible taxonomic level. Ants (Formicidae) were the best-represented prey group, accounting for 81% of prey, followed by mites (Oribatida: 8.8%; Mesostigmata: 3.5%), while ticks (Ixodidae) and snails (Gastropoda) showed values below 0.07%. Whereas we retrieved larger prey amounts by volume from toads with a larger snout–vent length, larger prey volume did not represent larger prey richness. Larger prey volume did, however, represent a larger number of prey items. In contrast to adults, who feed mainly on beetles (Coleoptera) and ants, bees, and their relatives (Hymenoptera) the diet of toadlets is composed of mites and ants.

Keywords. Body size, Rhinella icterica, mites, diet, stomach flushing, Araucaria Plateau, Brazil

Introduction use a sit-and-wait strategy, others can be described as active foragers or use a strategy that lays somewhere in The Araucaria Plateau, located in the northeastern between these two opposing strategies (Caldart et al., portion of the state of Rio Grande do Sul, harbours more 2012). Some active foragers are able to ingest a large than 55 amphibian species (Kwet et al., 2010). While number of small preys in a very short time period. This several of them represent species adapted to forest is the case when a frog locates an ant trail and performs environments, others can be found in open grasslands so called “blitz-feeding” (Mo, 2015). While species that within the mosaic of forest and grassland patches found feed mainly on ants, mites, and termites can be regarded all over the plateau. Anuran amphibians are mostly as ant specialists, others that feed on less chitinised described as generalist predators that feed mainly arthropods can be classified as generalists, or even as on arthropods, molluscs, annelids, and even small non-ant specialists if they actively avoid preying on ants vertebrates (Solé and Rödder, 2009), but they possess (Toft, 1980). the capacity of distinguishing between prey types. Rhinella icterica is widely distributed through the This allows different degrees of specialization (Freed, Atlantic Forest of southern Brazil and can also be found 1982) and prey capture strategies: while most anurans in Argentina and Paraguay (Frost, 2016). This toad is considered to be a generalist predator, feeding mainly on beetles and ants, but until now studies have only focused on adults and juveniles (Sabagh and Carvalho e Silva, 2008). While adult toads of this species 1 Departamento de Ciências Biológicas, Universidade Estadual represent the largest native amphibian living on the de Santa Cruz, Ilhéus, 45662-900, Bahia, Brazil Araucaria Plateau, with snout–vent lengths of 100–140 2 Programa de Pós-Graduação em Biologia, Universidade do vale do Rio dos Sinos, São Leopoldo, 93022-000, Rio mm, recently-metamorphosed toadlets are very small Grande do Sul, Brazil (9–10 mm). No data on the diet of post-metamorphs * Corresponding author e-mail: [email protected] of this species are available, but due to the huge 444 Mirco Solé et al. difference between the size of adults and juveniles we trail. The temperature during the capture activity was hypothesized that juveniles use other dietary resources recorded as 22ºC. than adults. Feeding is a crucial life history aspect of After capture, toads were transferred to the laboratory any amphibian species, and diet studies may be used as at CPCN Pró-Mata where they were weighed using a a surrogate to assess environmental impacts in natural digital scale (Voltacraft model PS-200 HTP) and had areas and habitat modifications (Anderson, 1991; Solé their snout–vent lengths (SVL) and mouth widths and Rödder, 2009). We therefore resolved to investigate (MW) measured using digital callipers. To retrieve the diet of R. icterica toadlets in order to gain a better stomach contents, a stomach flushing procedure was understanding of their trophic position in their native applied following Solé et al. (2005) with the exception habitat. of the amount of water used, which was reduced to 20 ml. We used a 60 ml syringe, a spatula, a number 10 Material and Methods urethra tube, forceps, 70% ethanol, and 2-ml Eppendorff tubes. The mass of the stomach contents was measured The study was undertaken at the Centro de Pesquisas (length and width) using digital callipers and stored in e Conservação da Natureza – Pró-Mata (CPCN-Pró- Eppendorff tubes. Stomach contents were identified Mata), an Atlantic Rainforest area maintained by the under a stereomicroscope to the lowest possible Pontifícia Universidade Católica do Rio Grande do taxonomic level. The toads were released one night later Sul, located on the Serra Geral, in São Francisco de at the same location where they were captured. Paula Municipality, Brazil (ca. 29.4667°S, 50.1667°W; To assess the stomach content volume we followed elevation 900 m). Griffiths and Mylotte (1987) where L = length and W Post-metamorphic Rhinella icterica were collected by = width. hand along a trail that encircles a pond where tadpoles 2 4 ⎛ L ⎞⎛W ⎞ of this toad were easily spotted, thus providing evidence V = p⎜ ⎟⎜ ⎟ 3 ⎝ 2 ⎠⎝ 2 ⎠ that this species reproduces there. Five researchers walked along the trail on 8 January 2015 between 0900 We assessed the relation between SVL and the volume and 1100 h and captured toadlets moving along the of stomach contents, and between stomach content and

Table 1 (Individuals 1–18). Prey categories retrieved from the stomachs of post-metamorphic individuals of Rhinella icterica at the Centro de Pesquisa e Conservação da Natureza Pró-Mata, Rio Grande do Sul, Brazil.

Prey items Individuals of R. icterica 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 MOLLUSCA Gastropoda ARTHROPODA Arachnida Araneae 3 1 2 1 3 3 1 Ixodidae 1 Mesostigmata 1 12 1 1 3 1 5 2 1 Oribatida 2 6 2 3 10 7 1 1 3 7 15 1 6 1 1 2 1 Prostigmata 1 2 Insecta Coleoptera 4 1 4 1 4 1 4 1 1 1 2 Collembola 1 2 1 1 2 2 1 2 1 Diptera 1 1 Formicidae 16 37 32 29 28 44 15 23 5 66 10 42 40 15 58 30 25 20 Hemiptera 1 1 2 1 Hymenoptera 1 2 Lepidoptera Abundance 22 51 38 30 37 73 25 26 15 80 22 64 43 26 62 32 29 21 Diet of post-metamorphic Rhinella icterica from Brazil 445

Table 1 Continued (Individuals 19–35).

Prey items Individuals of R. icterica TOTAL 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 MOLLUSCA Gastropoda 1 1 ARTHROPODA Arachnida Araneae 1 1 1 17 Ixodidae 1 Mesostigmata 3 2 1 1 4 1 4 1 1 45 Oribatida 1 1 1 3 3 4 1 3 1 1 2 1 8 13 1 113 Prostigmata 2 5 Insecta Coleoptera 2 1 1 3 1 1 1 1 35 Collembola 1 1 15 Diptera 2 Formicidae 4 17 40 11 24 30 22 31 25 39 64 25 75 45 5 26 27 1045 Hemiptera 5 Hymenoptera 2 1 1 7 Lepidoptera 2 2 Abundance 7 22 43 15 29 37 26 34 26 44 66 27 85 49 17 41 29 1293

prey abundance/richness. All analyses were submitted was explained by the SVL (F1,33 = 13.468; P < 0.001; to a simple linear regression using Systat 12 (Systat, Fig. 1). The volume of stomach contents in relation 2007). to the prey richness was not significantly correlated

(F1,33 = 1.031; P = 0.317), but the volume of stomach Results contents was significantly correlated in relation to prey abundance in stomachs (F = 6.266; P = 0.017; Fig. We collected 43 individuals of R. icterica and retrieved 1,33 2). a total of 1293 prey items from 35 stomachs; we classified prey items into 13 categories (Table 1). The remaining Discussion eight stomachs showed stomach contents, but the advanced stage of digestion did not allow identification We identified 13 prey categories for post-metamorphic of prey items. Some plant material was also retrieved R. icterica. In a study undertaken in the highlands of but is not shown due to its low abundance. the National Park of Itatiaia in the Rio de Janeiro State, Formicidae turned out to be the best-represented prey Sabagh et al. (2008) found a narrower prey range, group for post-metamorphic toads (n = 1045; 81%), composed basically of representatives from five orders. followed by mites belonging to the suborder Oribatida This could be linked to a decrease of invertebrate (n = 113; 8.8%) and the order Mesostigmata (n = 45; species at higher altitudes (Leakey and Proctor, 1987; 3.5%). Ixodidae and Gastropoda showed the lowest Brühl et al., 1999; Almeida-Neto et al., 2006). However, values (both n = 1; 0.07%). we need to note that prey availability was not measured To assess the relationship between SVL and the during our study. volume of stomach contents all 43 individuals were The observed dominance of ants in our study was used, whereas to assess the relation between stomach expected as such findings were also reported for other content and abundance/richness only 37 individuals were closely related toad species (Maia-Carneiro et al., 2013). used. The relationship between SVL and the volume However the high frequency of mites retrieved from the of stomach contents was significantly and positively stomachs represents new data in the characterization correlated. According to the determination coefficient of the species’ diet, representing approximately 13% (R2), 38.7% of the variation of stomach volume content of all identified prey. In a study with the congener R. 446 Mirco Solé et al.

Figure 1. Linear regression between SVL (in mm) and volume Figure 2. Linear regression between invertebrate abundance (in mm³) for stomach contents of Rhinella icterica collected at and volume (in mm³) for stomach contents of Rhinella icterica CPCN – Pró-Mata, Brazil. collected at CPCN – Pró-Mata, Brazil.

arenarum the high frequency of ants in the diet was and the Mesostigmata, which live as predators in the associated with the high availability of that prey type in leaf-litter and feed mostly on other arthropods (Walter the habitat (Menendez-Guerrero, 2001). The ingestion and Proctor, 2013). In studies that do not differentiate of plant material is probably an incidental aspect of mites into narrower taxonomic categories, it is hard invertebrate prey capture (Santos et al., 2004; Mahan to tell if mites were actively sought as prey or if they and Johnson, 2007; Isaacs and Hoyos, 2010). were ingested incidentally while they were parasitizing Amphibians are the group of vertebrates that ingests primary prey, such as grasshoppers or beetles. They the largest number of mites in trophic chains (Pengilley, could also represent secondary or tertiary prey from 1971; Norton and MacNamara, 1976; Stewart and the stomach of invertebrates that feed on mites, such Woolbright, 1996; Utzeri et al., 2004; Walton and as coleopterans of the families Ptiliidae, Pselaphidae, Steckler, 2005; Mead and Boback, 2006; Walton et al., Scydmaenidae, and Staphylinidae (Douglas, 2007). 2006). Small frogs and toads are more likely to feed We conclude that the volume of stomach contents on mites than salamanders. Pengilley (1971) found is proportional to the body size of the studied post- that four out of five species of frogs from the southern metamorphic toads and that there is trophic niche highlands of South Wales with SVL < 35 mm fed on overlap between adults, juveniles, and toadlets. mites (4–100% of the intestinal contents). Likewise, all However, whereas adults and toadlets share ants as 109 females of Oophaga pumilio reported by Donnelly a common prey in their diets, which likely causes no (1991) in Costa Rica contained mites in their stomachs, significant resource limitation issues or competition representing 38% of their diet. Most alkaloids in poison given the great abundance of ants, differences include frogs are linked to the ingestion of oribatid mites and the ability of adults to ingest large numbers of beetles, ants (Saporito et al., 2012). Most studies on toad diet whereas toadlets supplement their ant diet by mainly (e.g., Vanegas-Guerrero et al., 2016) have treated mites ingesting mites. as an inclusive prey category. Ours is the first study that identifies mites to lower taxonomic levels. The studied Acknowledgements. We thank Alexandro Marques Tozetti toads fed on several groups of mites, among which the and the “Mestrado e Doutorado em Biologia” from UNISINOS Oribatida can be characterized as decomposers of organic University for support and Hinrich Kaiser for excellent comments matter (Wehner et al., 2014), which are crucial for the on the manuscript. Instituto Chico Mendes de Conservação da recycling of nutrients in the leaf litter of tropical forests, Biodiversidade (ICMBio) issued the collecting permit (13708-1). Diet of post-metamorphic Rhinella icterica from Brazil 447

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Accepted by Hinrich Kaiser